Download Upper Mantle Seismic Anisotropy Beneath the West Antarctic Rift

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Upper Mantle Seismic Anisotropy Beneath the
West Antarctic Rift System and Surrounding
Region from Shear Wave Splitting Analysis
Natalie J. Accardo
Washington University in St. Louis
Douglas A. Wiens
Washington University in St. Louis
Stephen Hernandez
University of California, Santa Cruz
Richard C. Aster
Colorado State University - Fort Collins
Andrew Nyblade
The Pennsylvania State University
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Recommended Citation
Accardo, N.J. et al. (2014). Upper mantle seismic anisotropy beneath the West Antarctic Rift System and surrounding region from
shear wave splitting analysis. Geophysical Journal International 198(1), 414-429. DOI: 10.1093/gji/ggu117
This Article is brought to you for free and open access by the College of the Sciences at ScholarWorks@CWU. It has been accepted for inclusion in
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Authors
Natalie J. Accardo, Douglas A. Wiens, Stephen Hernandez, Richard C. Aster, Andrew Nyblade, Audrey D.
Huerta, Sridhar Anandakrisnan, Terry Wilson, David S. Heeszel, and Ian W.D. Dalziel
This article is available at ScholarWorks@CWU: http://digitalcommons.cwu.edu/geological_sciences/11
Geophysical Journal International
Geophys. J. Int. (2014) 198, 414–429
Advance Access publication 2014 May 21
GJI Seismology
doi: 10.1093/gji/ggu117
Upper mantle seismic anisotropy beneath the West Antarctic Rift
System and surrounding region from shear wave splitting analysis
Natalie J. Accardo,1,∗ Douglas A. Wiens,1 Stephen Hernandez,2 Richard C. Aster,3
Andrew Nyblade,4 Audrey Huerta,5 Sridhar Anandakrishnan,4 Terry Wilson,6
David S. Heeszel7 and Ian W. D. Dalziel8
1 Department
of Earth & Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA. E-mail: [email protected]
of Earth & Planetary Sciences, University of California, Santa Cruz, CA 95064, USA
3 Geosciences Department, Colorado State University, Fort Collins, CO 80523, USA
4 Department of Geosciences, The Pennsylvania State University, University Park, PA 16801, USA
5 Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA
6 Department of Geological Sciences, The Ohio State University, Columbus, OH 43210, USA
7 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92037, USA
8 Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA
2 Department
SUMMARY
We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave
splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37
broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue
technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked
to determine the best fitting splitting parameters for each station. Overall, fast anisotropic
directions are oriented at large angles to the direction of Antarctic absolute plate motion in
both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from
shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent
reflects multiple mantle regimes. We suggest that the observed anisotropy along the central
Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of
the largest zones of extended continental crust on Earth, results from asthenospheric mantle
strain associated with the final pulse of western WARS extension in the late Miocene. Strong
and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred
extension direction, a result unlike reports from the East African rift system and rifts within
the Basin and Range, which show much greater variation. We contend that ductile shearing
rather than magmatic intrusion may have been the controlling mechanism for accumulation
and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie
Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but
shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozenin’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea
appear to be radial to the dome and may indicate radial horizontal mantle flow associated
with an MBL plume head and low upper mantle velocities in this region, or alternatively
to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern
end of the WARS.
Key words: Seismic anisotropy; Dynamics of lithosphere and mantle; Antarctica.
∗ Now
414
at: Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA.
C
The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 16, 2016
Accepted 2014 March 25. Received 2014 March 15; in original form 2013 October 26
Seismic anisotropy beneath W. Antarctica
1 I N T RO D U C T I O N
Inferences on mantle flow processes beneath West Antarctica provides unique constraints on the deformational history of the geology
that underlies and strongly influences the West Antarctic Ice Sheet
(e.g. Winberry & Anandakrishnan 2004; Bingham et al. 2012).
2 GEOLOGIC SETTING
Antarctica is naturally divided into two distinct major provinces:
East Antarctica and West Antarctica. East Antarctica is comprised
of a large stable continental craton composed mainly of Precambrian
basement rocks that are uncomformably overlain by sedimentary
units. In contrast, West Antarctica comprises a number of discrete
mountainous crustal blocks that moved relative to East Antarctica
and each other during broad extension dating back to the Jurassic
(Dalziel & Elliot 1982; Dalziel 1992; Anderson 1999). They were
derived from the margin of the East Antarctic craton and from the
Pacific convergent margin. The TAM, the largest non-compressional
mountain belt in the world, separates the two provinces (e.g. ten
Brink et al. 1997).
2.1 The Transantarctic Mountains
The TAM are a gently tilted to block-faulted mountain range characterized by an absence of folding or thrust faulting. They extend
for over 3500 km from the Ross Sea to the Weddell Sea, prominently separating the Ross Embayment, a vast submerged region
of extended continental crust that is partially covered by Earth’s
largest ice shelf, from the icecap-covered Wilkes subglacial basin
(ten Brink et al. 1997). The TAM owes its earliest origins to the Neoproterozoic Beardmore Orogen and was later intensely deformed by
the early Paleozoic Ross Orogeny (Borg et al. 1990). The exact timing of the uplift(s) of the present TAM remains poorly constrained.
Apatite fission track thermochronology from Victoria Land and the
South Pole indicates that separate blocks of the TAM have experienced varying amounts of uplift and erosion at different times,
ranging from Early Cretaceous to Cenozoic (Fitzgerald 1992, 1994;
Stump & Fitzgerald 1992; Balestrieri et al. 1994). The relationship between the formation of the TAM and the WARS remains
unclear despite their obvious spatial relationship (Fig. 1). Major
WARS extension during the Cretaceous was apparently accompanied by relatively little denudation in the TAM, whereas limited
extension during the Cenozoic was accompanied by large amounts
of denudation (Karner et al. 2005; Huerta & Harry 2007). Geodynamic models of the WARS evolution suggest that the TAM may be
the abandoned margin of a Mesozoic West Antarctic plateau, and
the Cretaceous through Cenozoic crustal cooling ages of the TAM
record initial subsidence of the WARS region followed by fluvial
and glacial denudation of the mountain range (Bialas et al. 2007;
Huerta & Harry 2007).
2.2 The West Antarctic Rift System
The WARS abuts the TAM and is generally characterized by a topographic trough 750–1000 km wide and 3000 km long (e.g. Behrendt
et al. 1991). It is often described as an asymmetric rift system that
is proposed to run from the Ellsworth-Whitmore mountains crustal
block (EWM) to the edge of the Ross Embayment where it meets
northern Victoria Land (Fig. 1). The region is characterized by
a regional positive Bouguer gravity anomaly that extends from the
Ross Sea throughout the Byrd Subglacial Basin (Behrendt & Cooper
1994). Uncertainty still persists on the timing and evolution of the
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Seismic anisotropy, the dependence of seismic velocities on propagation and polarization direction, has emerged as one of the best
indicators of both past and present mantle deformation and flow
(e.g. Silver & Chan 1988; Fischer et al. 1998; for a review see
Long and Silver 2009). Mantle deformation often leads to seismic
anisotropy; either through lattice preferred orientation (LPO) of
anisotropic minerals or through shape preferred orientation (SPO)
of materials having different seismic velocity. Because of the link
between deformation and anisotropy, the mapping of anisotropic
structure can yield some of the most direct constraints available on
convection, deep tectonically induced deformation and generally
on the coarse- and finer-scale fabric of Earth’s mantle (Silver 1996;
Savage 1999; Long & Becker 2010).
Olivine constitutes over 65 per cent of the upper mantle and,
because it has a large single-crystal anisotropy (∼18 per cent; e.g.
Mainprice et al. 2005), it is thought to make the primary contribution to the observed anisotropy. Laboratory studies of artificially
deformed olivine aggregates indicate that multiple deformational
fabrics (A-, B-, C-, D- and E-type) for olivine exist (for a review
see Karato et al. 2008). The well-known A-type fabric promotes
the alignment of the olivine fast axis with the direction of maximum shear, which may be in the extension and/or flow direction
(Zhang & Karato 1995). Conversely, B-type fabric (favoured by
high-stresses, low temperatures and the presence of water) predicts
that fast axes will align normal to the direction of maximum shear
(Jung & Karato 2001). Experimental studies have shown that the
development of olivine fabrics depends greatly on the conditions
of deformation, including, stress, water content, temperature and
pressure (e.g. Katayama et al. 2004; Mainprice et al. 2005). There
are multiple models that explain the presence of mantle seismic
anisotropy, including: (1) mantle anisotropy induced by ongoing
extension/compression due to the LPO of olivine, (2) anisotropy
due to the alignment of parallel dikes or melt-filled lenses, (3) fossilized anisotropy in the lithospheric mantle due to LPO from past
tectonic events. The detailed interpretation of anisotropic observations in terms of stress, strain, hydration, temperature and composition throughout the mantle is a complex and active area of research
(e.g. Long & Silver 2009). Thus, while anisotropic fabrics are inherently complex, they provide unique information on the fabric of
the mantle and hence on its deformational history and present state.
To date, very few seismic investigations have probed the mantle
beneath Antarctica due to the harsh conditions of working there
and the only recently resolved issues of maintaining seismographic
stations there. Furthermore, no prior studies have examined the
anisotropic nature of the West Antarctic Rift System (WARS),
one of the largest regions of extended continental crust on Earth
(Behrendt et al. 1991). Thus, determining the anisotropic character
of the WARS has much to contribute to our understanding of mantle deformational fabrics associated with continental rifts, which
are shown to vary significantly between different continental rift
settings (e.g. Vinnik et al. 1992; Gao et al. 1997; Kendall et al.
2005; Obrebski et al. 2006; Wang et al. 2008; Eilon et al. 2014).
This study examines the seismic anisotropy beneath West Antarctica using shear wave splitting of SKS, SKKS and PKS phases obtained from 37 broad-band seismometres deployed in the WARS and
throughout the rest of West Antarctica, including Marie Byrd Land
(MBL) and in the Transantarctic Mountains (TAM). These data allow us to better understand the deformational history and mantle
flow patterns of a largely unexplored continental region where geologic exposures occupy less than 2 per cent of the surface area.
415
416
N. J. Accardo et al.
WARS because of extensive ice cover and limited number of geological and geophysical studies conducted in this region. The total
extension accommodated by the WARS crust is poorly constrained,
and estimates vary widely. Busetti et al. (1999) estimate 120–
250 km, Grindley & Oliver (1983) estimate 200–500 km, numerous
authors (e.g. Fitzgerald et al. 1986; Behrendt & Cooper 1994) estimate 255–300 and Trey et al. (1999) estimate 480–500 km. Recent
studies show that portions of the WARS are underlain by extremely
thin continental crust, as thin as ∼20 km near the Bentley Trench,
in the vicinity of Ross Island and the Byrd Basin (Winberry &
Anandakrishnan 2004; Chaput et al. 2014), indicating significant
localized crustal thinning separating the East Antarctic craton and
the EWM crustal block from the MBL and Thurston Island crustal
blocks.
Studies suggest extension has occurred in two major pulses, with
the first during the Jurassic-Cretaceous, inducing major distributed
crustal thinning over the entire WARS. A second pulse later in
the Cenozoic is widely documented in the Ross Sea region (e.g.
Behrendt 1999; Karner et al. 2005; Wilson & Luyendyk 2006).
Cenozoic extension may thus have itself occurred in two stages,
with an earlier period from 41 to 26 Ma concentrated in the Ross
Sea region and a Miocene pulse affecting mostly regions further
inland (Granot et al. 2010; Granot et al. 2013). Whether or not the
rifting occurred in several pulses, it is generally accepted that the
major extension of the region commenced ca. 105 Ma (Luyendyk
1995; Siddoway et al. 2004) and slowed by ca. 17 Ma (Cande et al.
2000; Hamilton et al. 2001; Granot et al. 2010). Presently, the low
level of seismicity (Winberry & Anandakrishnan 2004) and GPS
measurements indicating low strain rates between East and West
Antarctica (e.g. Donnellan & Luyendyk 2004; Wilson et al. 2011)
suggest that the rift is currently inactive or extending very slowly.
The WARS differs from other continental rifts in several important respects. The diffuse Jurassic-Cretaceous extension of the
WARS affected a much wider region relative to most continental
rifts (Huerta & Harry 2007), such as the Baikal Rift, East African
Rift and North American Midcontinent Rift (Logatchev et al. 1983;
2.3 The Ellsworth-Whitmore Mountains crustal block
and Pensacola Mountains
The EWM, located at the head of the Weddell Sea embayment, represent a geologically and geophysically distinct crustal block within
West Antarctica (Dalziel & Elliot 1982). The EWM are anomalous
in stratigraphy, structural grain and deformational history relative
to their surroundings. Geologic and palaeomagnetic investigations
strongly indicate that the EWM underwent extensive translation and
rotation from their initial location prior to the breakup of Gondwana
(e.g. Clarkson & Brook 1977; Watts & Bramall 1981; Curtis 2001;
Randall & Niocaill 2004). The stratigraphy of the EWM does not
correlate with that of any other part of West Antarctica but instead
exhibits obvious ties to the Gondwana craton margin sequences seen
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Figure 1. POLENET/ANET station locations (red circles) used in this study
and geologic regions of interest; WARS, West Antarctic Rift System; EWM,
Ellsworth-Whitmore Mountains crustal block; PCM, Pensacola Mountains;
MBL, Marie Byrd Land; THR, Thurston Island crustal block. The boundaries for the East Antarctic Craton, the EWM crustal block, and the MBL
crustal block from Dalziel & Elliot (1982) are drawn in blue. Background
colour scale shows bed elevation beneath the ice sheets (Fretwell et al. 2013).
Black bar indicates the scale of 500 km.
Ingersoll et al. 1990; Keller et al. 1991). Also the extremely deep
WARS rift floors are at much lower elevations than other continental rift zones, even when deglaciated and rebound is considered (LeMasurier 2008). The WARS crust is generally thin, with
aforementioned areas of very thin crust beneath much of the West
Antarctic Ice Sheet, increasing to a maximum near 30 km in western MBL (Chaput et al. 2014). Comparisons of rift floor elevations
in conjunction with the unique extensional history of the WARS
show that the system does not conform to the behaviour expected
from previously investigated continental rifts or broad regions of
extension like the U.S. Basin and Range.
Cenozoic bimodal alkali volcanic rocks, commonly found within
continental rifts, characterize the MBL shoulder of the WARS (e.g.
LeMasurier 1990), and aeromagnetic surveys suggest the presence
of these rocks over an area of >5 × 105 km2 within the rift itself. The
absence of abundant exposures makes it difficult to estimate when
the bulk of late Cenozoic volcanic rocks erupted. Nevertheless,
available radiometric ages extend from the present back to 30 Ma
(LeMasurier 1990). An active mantle plume (Behrendt et al. 1991),
intraplate tectonic displacements induced by the plate circuit of the
Southern Hemisphere (Rocchi et al. 2002), and thermal perturbation of metasomatized lithosphere (Finn et al. 2005) have all been
proposed as mechanisms of magma generation and emplacement,
although these hypotheses remain largely untested.
The extent of the WARS towards the Antarctic Peninsula remains
conjectural, as most geophysical campaigns to date have concentrated on the more accessible and better geologically exposed Ross
Sea region (e.g. Behrendt et al. 1996; Trey et al. 1999; Karner
et al. 2005). However, it seems likely from ice rebound-adjusted
topography (e.g. Chaput et al. 2014) that the WARS extensional
zone continues to the Pacific coast in the Amundsen Sea region.
Subglacial basins in the Bentley Trench/Amundsen Sea region are
comparable in lateral extent to those within the Ross Sea, yet reach
much greater surface depths. The deepest portions of these basins
extend more than 1500 m below sea level, making them the lowest
elevation continental topographic features on Earth. These basins
and troughs likely accommodated lateral motion during the EoceneOligocene when the region underwent a period of significant convergence (Granot et al. 2013). Recent aeromagnetic and aerogravity
surveys have identified rifts in the Pine Island and Ferrigno areas,
both locations of gravity and magnetic anomalies consistent with
other recently active rifts, that are mechanically related to the central
WARS (Jordan et al. 2010; Bingham et al. 2012). These features
show evidence for only small amounts of sediment infill, suggesting that they may have only opened during the last pulse of WARS
extension in the Neogene (Bingham et al. 2012).
Seismic anisotropy beneath W. Antarctica
2.4 The Marie Byrd Land Dome
The MBL intraplate volcanic province is a large region defined
by a 1000 × 550 km dome that broadly rises to approximately
2000 m in deglaciated surface elevation and is punctuated by volcanoes reaching deglaciated elevations over 4000 m. The main MBL
dome is approximately centred on a belt of mid-to-late Cenozoic
alkaline volcanoes that extends along Antarctica’s Pacific Coast.
The MBL province is one of two regions within this belt where
large central vent volcanoes with more than 2000 m of relief are
found (LeMasurier & Rex 1989). The WARS lies polewards of MBL
and separates it from the TAM and the EWM. Crustal thicknesses
(26–28 km) beneath the volcanic province are 2–5 km greater than
they are within much of the WARS (Chaput et al. 2014). Thinner
crust is also found to the east of MBL in the region of Pine Island
Glacier (Jordan et al. 2010; Chaput et al. 2014).
Lines of volcanoes divided by subglacial basins characterize
the landscape within the MBL dome (LeMasurier & Rex 1989).
The Cenozoic volcanic deposits of the MBL overlie mid- and preCretaceous rocks. The exposed Jurassic and Lower Cretaceous rocks
in the MBL dome region are mostly igneous I-type granites and related volcanic rocks, while the mid-Cretaceous rocks are A-type
granitoids thought to have formed at mid-crustal levels and then
exposed during extension before the breakup of Gondwana in this
region. These formations are thought to reflect a transition between
subduction- to rift-related magmatism prior to the separation of the
New Zealand microcontinent (Weaver et al. 1994). Volcanic activity
continued in the Holocene (i.e. Dunbar et al. 2008) and recent studies have identified deep earthquakes polewards of Mount Sidley and
Mount Waesche, indicating that the region remains magmatically
active today (Lough et al. 2013).
Prior to Gondwana breakup, MBL sat between East Antarctica and New Zealand, where the Phoenix Plate was subducting.
The separation of New Zealand from the Antarctic core of Gondwana in the Late Cretaceous was the last in a series of fragmenta-
tion events during the breakup of the supercontinent (Storey 1995).
Granites from the Ruppert and Hobbs coasts of MBL indicate a prolonged period of subduction-related magmatism beginning at 320
± 3 Ma or earlier (Mukasa & Dalziel 2000). Subduction ceased
at ca. 108 Ma as a result of either collision of the Pacific-Phoenix
spreading ridge with the subduction zone (Bradshaw 1989) or abandonment of the spreading ridge by slab capture (Luyendyk 1995).
After subduction stopped, plate boundary forces changed dramatically and separation accelerated. Evidence for ocean floor development between New Zealand and MBL by 81 Ma suggests that
complete separation of the continental blocks had occurred by that
time (Mukasa & Dalziel 2000).
Many authors have proposed the existence of a mantle plume
beneath MBL beginning in the Cenozoic (LeMasurier & Rex 1989;
Behrendt et al. 1991; Hole & LeMasurier 1994; Sieminski et al.
2003) to account for the intraplate MBL Cenozoic alkaline basaltic
province. It has also been suggested that a mantle plume might
have existed beneath MBL beginning in the mid-Cretaceous and
coincident with initial WARS opening (Storey et al. 1999) but this
earlier plume hypothesis is more controversial. Mantle plumes have
been linked to lithospheric extension in both the East African rift
(e.g. Schilling 1973a; Ritsema et al. 1999) and the Reykjanes Ridge
(e.g. Schilling 1973b; Gaherty 2001) and thus could be expected to
potentially play a role within the WARS. High heat flow measurements in the Ross Sea (66–114 mW m−2 ; Blackman et al. 1987)
and at the West Antarctic Ice Sheet drill site (240 mW m−2 ; Clow
& Cuffey 2012), limited Cenozoic extension that, to first order,
seems unable to account for the volume of observed volcanism,
and geochemical similarity between the MBL basalts and oceanic
island basalts can be taken to be supportive of the plume hypothesis
(LeMasurier & Rex 1989; Hole & LeMasurier 1994).
3 D ATA A N D M E T H O D S
3.1 Data
Data from 37 broad-band seismic stations installed throughout West
Antarctica between 2007 and 2011 as part of the POLENET/ANET
project are examined here. Table 1 gives station names and locations.
Many of the stations form a part of the POLENET/ANET backbone
array, and most are co-located with GPS sensors near nunataks and
larger rock outcrops. A number of temporary stations deployed for
2 yr (station names beginning with ST and hereafter referred to
as the POLENET transect) operated for a shorter time than backbone stations in a line extending from the Whitmore Mountains,
through the WARS, and across the MBL dome. Fig. 1 shows station
locations as well as prominent geologic and geographic features
within West Antarctica. Reliable year-round station operation was
accomplished using a combination of lead-acid batteries, specially
insulated enclosures, and winter-use primary lithium batteries designed and supported by the IRIS PASSCAL program (Nyblade
et al. 2012).
Earthquake sources used for this study were selected for USGS
NEIC-determined Mw > 6.0 and epicentral distances between 90◦
and 140◦ . SKS, SKKS and PKS phases were carefully examined,
and only those with a sufficient signal-to-noise ratio (SNR) and that
were clearly separated from other phases were chosen for analysis.
The highest SNR seismograms were analysed without filtering;
these were generally evaluated as the highest quality results. Records
with lower SNR were bandpass filtered with a low frequency corner
of 0.02 Hz and a variable high frequency corner between 0.15 and
0.5 Hz.
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within the TAM (Schopf 1969). The structural trend of the EWM
strikes close to transverse to both the TAM and, in a reconstructed
Gondwana supercontinent, the Cape Fold Belt in Africa. Beginning
in the 1960s, numerous authors have suggested that these enigmas can be resolved if the EWM originated at a location adjacent
to the palaeo-Pacific margin of Gondwana, between South Africa
and the Coats Land coast of Antarctica, and later underwent lateral
translation and rotation along the Antarctic margin during Gondwana breakup (e.g. Schopf 1969; Dalziel 1992; Goldstrand et al.
1994). More recent palaeomagnetic and stratigraphic investigations
have largely confirmed that the EWM previously sat adjacent to
South Africa in a proposed continental rift basin prior to Gondwana breakup (Curtis 2001; Randall & Niocaill 2004). The EWM
crustal block composes thicker crust and seismically faster upper
mantle than the surrounding WARS (Heeszel et al. 2011; Lloyd
et al. 2012; Chaput et al. 2014), consistent with the idea that it is
indeed a fragment of older pre-extensional continental lithosphere.
The Pensacola Mountains (PCM) lie between the northern TAM
termination and the Weddell Sea margin (Fig. 1). The PCM show
many geologic features common along the TAM, such as late Neoproterozoic to Late Paleozoic magmatic and sedimentary sequences
deformed during a series of Neoproterozoic to early Paleozoic orogenic events related to the end of the Ross Orogeny (Stump 1995;
Rowell et al. 2001). In a large-scale sense, the PCM are a continuation of the TAM, also representing an extensional uplift of the older
orogeny formed along the edge of the East Antarctic Craton.
417
418
N. J. Accardo et al.
Table 1. Summary of anisotropic parameters determined in this study. Grid fast direction indicates the fast direction relative to a rectangular Grid stereographic
convention where north is taken to be along the prime meridian, south becomes the 180◦ meridian, east becomes 90◦ E, and west becomes 270◦ E. Values in
parentheses under the ‘Phases Used’ column indicate the number of null observations reported for each station. Null observations were not utilized when
stacking for the final splitting parameters.
Latitude
Longitude
Geographic fast
direction (degree)
Phi standard
error (+/−)
Grid fast
direction (degree)
Splitting
time (s)
Splitting time
standard error (+/−)
Phases used
Grade
BEAR
BYRD
CLRK
DEVL
DNTW
DUFK
FALL
FISH
HOWD
KOLR
LONW
MECK
MILR
MPAT
PECA
SILY
SIPL
ST01
ST02
ST03
ST04
ST06
ST07
ST08
ST09
ST10
ST12
ST13
ST14
SURP
THUR
UNGL
UPTW
WAIS
WHIT
WILS
WNDY
−74.548
−80.017
−77.323
−81.476
−76.457
−82.862
−85.307
−78.928
−77.529
−76.155
−81.347
−75.281
−83.306
−78.030
−85.612
−77.133
−81.641
−83.228
−82.069
−81.407
−80.715
−79.332
−78.639
−77.948
−76.531
−75.814
−76.897
−77.561
−77.838
−84.720
−72.530
−79.775
−77.580
−79.418
−82.682
−80.040
−82.370
−111.851
−119.473
−141.849
161.975
−107.780
−53.201
−143.628
162.565
−86.769
−120.728
152.735
−72.185
156.252
−155.022
−68.553
−125.966
−148.956
−98.742
−109.124
−113.150
−116.578
−121.820
−123.795
−125.531
−128.473
−129.749
−123.816
−130.514
−134.080
−171.202
−97.561
−82.524
−109.040
−111.778
−104.387
−80.559
−119.413
61
−38
7
25
−84
−42
−2
53
−65
–
−75
–
–
21
−70
−42
−18
−72
−42
−37
−38
–
−13
−17
−36
−39
−55
−5
–
22
73
–
−64
−29
−58
−76
−42
5
2
13
15
1
0
2
15
4
–
5
–
–
7
1
3
1
3
2
0
1
–
4
5
2
1
5
3
–
1
4
–
0.15
1
9
2
3
−51
22
45
6
−12
84
34
35
28
–
77
–
–
45
41
12
13
9
28
29
25
–
43
37
15
11
1
44
–
30
−25
–
6
39
17
23
18
0.75
0.68
0.70
0.60
0.60
0.95
0.50
0.40
0.55
–
0.60
–
–
0.45
0.90
0.55
0.73
1.15
1.30
1.28
1.43
–
0.70
0.55
0.70
0.90
0.55
0.63
–
0.68
0.90
–
0.68
0.73
0.78
1.13
1.08
0.1125
0.0625
0.275
0.25
0.0375
0.0375
0.0375
0.1125
0.05
–
0.0625
–
–
0.0625
0.0375
0.0375
0.0375
0.0375
0.04
0.025
0.075
–
0.0625
0.075
0.05
0.05
0.05
0.0375
–
0.0375
0.0625
–
0.225
0.0375
0.225
0.075
0.1125
4 (2)
5 (3)
3 (0)
2 (2)
5 (3)
20 (2)
8 (2)
4 (2)
9 (2)
–
4 (0)
–
–
4 (1)
10 (0)
11 (0)
15 (3)
7 (3)
12 (2)
11 (3)
5 (1)
–
5 (6)
6 (2)
5 (0)
3 (3)
4 (2)
5 (2)
–
11 (3)
5 (1)
–
3 (1)
13 (2)
3 (2)
5 (0)
3 (2)
B
B
B
B
B
A
A
B
A
C
B
C
C
B
A
A
A
A
A
A
A
C
B
A
B
B
B
B
C
A
B
C
B
A
B
B
B
3.2 Methods
We determine the anisotropic parameters using the widely-applied
method of Silver & Chan (1991) which aims to determine the appropriate splitting parameters via the minimization of energy on the
transverse component. This method analyses ground particle motion by calculating the covariance matrix of the horizontal components for all possible splitting directions and reasonable delay times.
The most linear-restored particle motion, and thus the preferred
fast anisotropy direction and delay time parameters for reversing
the effect of simple anisotropy at a given station, are found by
minimizing the magnitude of the smaller eigenvalue of the particle
motion covariance matrix. The process of picking windows around
the individual phases and then calculating the splitting parameters
was automated using the method of Teanby et al. (2004). This algorithm performs splitting analysis on a range of window lengths
and then finds those measurements that are stable over many different windows. These windows are then placed into clusters, with
the final window chosen from the splitting analysis with the lowest
error in the cluster with the lowest variance. We then evaluate the
result from displays showing unrotated and rotated waveforms, the
ground particle motion and the misfit contour plot for the predicted
fast axis (Fig. 2).
Following the methodology of Silver & Chan (1991), the misfit
contour region is constructed from the α confidence level from the
following expression:
k
E t (φ, δt)
f k,n−k (1 − α),
≤1+
E tm
n−k
(1)
where Et (φ,δt) represents the n-point time-series containing the SKS
phase, k represents the number of parameters (in this instance φ &
δt), and f represents the inverse F probability distribution. For the
95 per cent confidence level α = 0.05.
All results were individually inspected and assigned a quality
rating of A, B or C based on several factors including: (1) SNR,
(2) linearization of particle motion, (3) waveform coherence between the two horizontal components rotated into the fast and slow
directions and (4) tightness of the misfit contours as a function of
splitting parameter. To be rated as A quality, measurements had to
Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 16, 2016
Station
Seismic anisotropy beneath W. Antarctica
419
satisfy certain criteria for each of the four factors described above.
Measurements had to have a SNR > 8 to satisfy criterion (1). A
bandpass filter was applied to noisy records with initially low SNR
to improve their quality but no distinction between unfiltered and
filtered records was made when judging criterion (1). To satisfy
criterion (2), measurements that showed elliptical particle motion
on the uncorrected seismogram had to show nearly linear particle
motion on the corrected seismogram.
To be rated as B quality, measurements had to have SNR > 3
to satisfy criterion (1). To satisfy criterion (2), measurements that
showed elliptical particle motion on the uncorrected seismogram
had to show a general reduction in ellipticity on the corrected seismograms. Criteria for A and B quality measurements for both waveform coherence and tightness of misfit contours were qualitatively
judged. Only results rated B or higher were included in the final
analyses.
Measurements judged to be ‘null’ were those that did not show
energy on the transverse component, and thus a near-radiallyoriented horizontal particle motion prior to analysis. Such events
often yield erroneously large delay times when analysed. Null measurements can result when the initial polarization of the incoming wave is parallel or orthogonal to the fast anisotropic direction,
when there is no anisotropy along the ray path, or when the SNR is
low. Null measurements are most confidently captured when multiple simultaneous shear wave splitting techniques are used (i.e.
the rotation-correlation method and the transverse minimization
method; Wustefeld & Bokelman 2007; Long & Silver 2009). Because we utilized a single method for our shear wave splitting
analysis, those measurements identified as null were not incorporated into the final stacked solution for splitting parameters. The
number of null measurements observed at each station is given
in Table 1.
A stacking method (Wolfe & Silver 1998) was used to increase
the robustness of the results relative to analysis of individual events.
This program produces a weighted sum of the individual misfit
surfaces and computes a global solution for each station. Only A
and B quality events were used in the stacking procedure.
Final stacked solutions for each station were assigned a quality
of A, B or C based on (1) the quality of the single events included in
the stack and (2) the backazimuth sampling of the station. Stations
are categorized as A if the global solution consists of at least four
individual measurements and are sampled only with single event
ratings of A. Stations are categorized as B if the global solution
consists of at least two individual measurement and is sampled
only with single event ratings of A or B. A quality stations are
plotted as thick vectors and B quality stations are plotted as thin
vectors in Fig. 3. Stations that did not meet the above criteria were
categorized as C. Note that comparison of splitting directions can
be difficult in geographic coordinates for stations near the South
Pole; for convenience we have also converted fast directions to the
rectangular Grid coordinate system, with north oriented along the
Prime Meridian and east along longitude 90◦ E, as is common for
work near the poles. We will refer to splitting directions using this
‘Grid’ coordinate system in the following discussion. Both Grid and
geographical directions are given in Table 1.
4 R E S U LT S
Stacked shear wave splitting parameters with a grade of B or better
were determined for 31 out of 37 stations of the POLENET/ANET
array. Six stations collected insufficient data or poor quality data and
were not used in the subsequent analysis and discussion. Table 1
shows the fast direction of anisotropy, delay time and associated
Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 16, 2016
Figure 2. Example SKS splitting analysis for a single, A quality, non-null event. (a) Event processing reduces the energy on the transverse component. (b)
The windowed waveforms and the particle motion plots for the raw data (left) and the data corrected for observed anisotropy (right). (c) Contour plot showing
misfit as a function of splitting parameter. (d) Contour plot for the stacked solution for the station with observed anisotropy parameters. Values on the contour
interval indicate the nσ value of the given contour. The contour labelled 2 indicates the 95 per cent confidence interval.
420
N. J. Accardo et al.
errors for each station. Also listed is the number of null observations
and stacked events used at each station as well as the final quality
rating for the station.
4.1 The TAM, EWM and PCM
The four stations located at the base of the TAM, near the boundary
between the TAM and the Ross Ice Shelf (FISH, DEVL, FALL and
SURP) show relatively consistent fast directions, ranging from Grid
6◦ to 35◦ (Fig. 3). The fast directions in the central TAM are oriented
roughly perpendicular to the TAM front. The fast direction at FISH
is consistent with fast directions found in the McMurdo Dry Valleys
area by Barklage et al. (2009). Station LONW, located at the crest
of the TAM further towards East Antarctica, shows a very different
orientation of Grid 77◦ .
Stations located in the EWM block (HOWD, WILS, WHIT and
ST01) show similar fast directions but the splitting magnitudes
are variable; fast directions range from Grid 9◦ to 28◦ and delay
times range from 0.55 to 1.15 s. Splitting magnitudes for the two
stations located in the PCM (PECA and DUFK) are uniform and
fast directions are variable. Fast directions at these locations are
Grid 41◦ and 84◦ and delay times are 0.90 and 0.95 s, respectively.
Both stations located in this region are A quality and are among the
best constrained results due to both the number of usable events as
well as the quality of those events.
4.2 The West Antarctic Rift System
Results from the seven stations located along and near the
POLENET transect show extremely similar fast directions and
regionally uniform splitting magnitudes (Fig. 4a). Stations ST02,
ST03 and ST04 show fast directions between Grid 25◦ and 29◦ and
splitting times between 1.28 and 1.43 s (Fig. 5). Together with station WNDY (Grid 18◦ , 1.08 s) somewhat to the west, they form an
area of remarkably large and consistent splitting, indicating strong
and uniform anisotropy across this region. Further north along the
transect, BYRD and ST07, as well as station WAIS to the east, show
similar if less tightly clustered splitting directions (Grid 22◦ –43◦ )
Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 16, 2016
Figure 3. Splitting results for West Antarctica. The region of interest is outlined in the green box on the inset map of Antarctica. Thick red vectors represent
A quality results and thin red vectors represent B quality results. Black vectors represent shear wave splitting results assembled from a variety of studies
(Müller 2001; Barklage et al. 2009; Hernandez et al. 2009). Only A quality results and results from stations with multiple high quality phases were included
from previous studies. Vector azimuths denote fast direction of the event stacks. The length of the vector is proportional to the splitting time, with splitting
time scale indicated at right. Given the close proximity of many seismic stations, we have only labelled station names not shown in Fig. 4. Grid directions allow
for simpler discussion of azimuthal values at high latitudes, and this map projection is aligned Grid N. Solid green arrows indicate the direction of Antarctic
absolute plate motion in the hotspot reference frame calculated using HS3-NUVEL; at 75◦ S 120◦ W APM has an azimuth of 281.6◦ W and a velocity of
1.89 cm yr−1 . In comparison, no-net rotation reference frames calculated for the same location show APM with an azimuth of 91◦ E and a velocity of 1.77 cm
yr−1 . Generally, fast directions across the West Antarctic Rift System are oriented roughly Grid NE-SW and splitting times average ∼1 s. Decreased delay
times and variable fast directions are notable in the vicinity of the MBL dome and volcanic province. Background colour scale indicates bedrock elevation from
Fretwell et al. 2013. Blue labels indicate locations of subglacial basins; PIR, Pine Island Rift; BSB, Byrd Subglacial Basin; BST, Bentley Subglacial Trench.
Seismic anisotropy beneath W. Antarctica
421
but smaller splitting times (0.68–0.73 s). The only station located
along the Siple coast, bordering the western Ross Ice Shelf (SIPL)
shows a fast direction of Grid 13◦ and delay time of 0.73 s, very
similar to that found along the transect.
Three stations (BEAR, DNTW and UPTW) are located east of the
MBL dome, on the margin of and within the Thwaites Glacier and
Pine Island Glacier region. These stations show consistent delay
times between 0.6 and 0.75 s but widely varying fast directions
(Grid −51◦ , −12◦ and 6◦ , respectively).
4.3 Marie Byrd Land
Overall, results across MBL show largely consistent delay times and
two distinct patterns of fast directions (Fig. 4b). Delay times for the
seven stations located in this region range from 0.55 to 0.90 s. The
largest delay times occur near the coast and the smallest occur on
the pole-ward portion of the dome, near the boundary of the WARS.
Fast directions for this area can be split into two distinct groups.
One set (ST09, ST10, ST12 and SILY), located near the crest of the
dome, shows fast directions clustered between Grid 1◦ and 15◦ . The
second set of stations (ST08, ST13 and CLRK), located south and
west of the dome, display fast directions between Grid 37◦ and 45◦ .
The station MPAT, located approximately where the Siple Coast,
Ross Ice Shelf and the Pacific coast come together, is east of the
MBL dome yet still within the MBL crustal block and shows a
similar splitting direction (Grid 45◦ ) with a smaller splitting time
(0.45 s).
5 DISCUSSION
5.1 Depth extent of the observed anisotropy
It is well recognized that the depth resolution of core phase splitting measurements is poor due to the path-integrated nature of
the measurements and the steep ray path incidence angles of core
phases. Tight constraints on the depth distribution of anisotropy can
be inferred from short period lateral variations via Fresnel zone
arguments whereby the depth of anisotropy is limited to regions
where Fresnel zones corresponding to different splitting observations should not overlap (Alsina & Sneider 1995). However, it is
also well established that anisotropic signals observed in teleseismic
shear wave splitting studies such as this primarily reflect anisotropy
in the upper mantle. Shallow sources of anisotropy may be produced from aligned cracks and microcracks, or fabric, in the upper
10–15 km of the crust. However, crustal shear wave splitting measurements generally range between 0.05 and 0.2 s (e.g. Aster et al.
1990; Savage 1999) and do not strongly affect the typically much
larger mantle anisotropy signatures for SKS signals. Furthermore,
this region is characterized by thin crust, generally 35 km or less,
which would likely be unable to accumulate an appreciable fraction
of the observed delay times (Winberry & Anandakrishnan 2004;
Chaput et al. 2014). Barruol & Mainprice (1993) showed that in
extreme cases where the entire crust is composed of anisotropic
material with steeply dipping foliations, the crustal signature of
anisotropy could reach up to 0.5 s. However, the complex tectonic
Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 16, 2016
Figure 4. Splitting results from the WARS and MBL displayed as in Fig. 3. (a) Results from the POLENET WARS-crossing transect. Within the WARS,
stations reveal larger splitting times with markedly uniform directions. Splitting is strongest (1.43 s) at station ST04, located over a rift basin, and weakens
as stations progress coast-wards to the MBL. (b) Results from MBL can be separated into two distinct groups. Observations Grid north of the dome (SILY,
ST12, ST08) have relatively small delay times (∼0.5 s) and varying fast directions. Observations on the main dome and closer to the coast (ST10; ST09) show
larger delay times (>0.6 s) and a consistent fast direction that is nearly Grid N–S. Stations located Grid northwest of this region (BEAR, DNTW, UPTW) have
consistent delay times (∼0.7 s) and widely varying fast directions that show a general radial pattern relative to the MBL dome.
422
N. J. Accardo et al.
or lower mantle anisotropy along a particular azimuth should be
down-weighted in the final, averaged result.
5.2 Relationship of anisotropy to absolute plate motion
over the mantle
Figure 5. Individual splitting results for all events stacked in the final solution for A-quality stations along the northern portion of the POLENET
transect (ST01, ST02, ST03, ST04). The BAZ of the event is represented
by angle clockwise from 0◦ , the delay time by the radial distance from the
origin of the plot, and the direction of the fast axis by the azimuth of the red
vector. Individual splitting results for the remaining stations are available in
the supplementary material.
history of the region suggests that retention of such large-scale
coherent crustal structure would be difficult. Growing evidence
suggests that anisotropy may be present in the lower mantle and
at the core-mantle boundary (e.g. Garnero et al. 2004; Panning &
Romanowicz 2006). However, consistency in splitting parameters
between rays with different backazimuths (thus different sampling
regions of the lower mantle) strongly suggests that such effects do
not contribute appreciably to these observations. Individual plots
of backazimuth against fast axis and delay time are included in the
Supporting Information, a subset of the stations are shown in Fig. 5.
Our final measurements are produced by stacking arrivals from a
variety of backazimuths when available, so that any bias from mid-
5.3 Anisotropy in East Antarctica versus West Antarctica
Splitting results from West Antarctica are highly distinct from those
reported for East Antarctica, particularly compared to the South Pole
region and the East Antarctic highlands (Müller 2001; Bayer et al.
2007; Reading & Heintz 2008; Barklage et al. 2009; Fig. 6). Fast
directions reported from the PCM and along the TAM (e.g. DUFK,
PECA and LONW) are also distinct from the overall Grid NE–SW
direction seen across much of West Antarctica. Mantle shear velocity maps show that these three stations lie outside of the low velocity
upper mantle that underlies most of West Antarctica (e.g. Heeszel
et al. 2011; Fig. 7). These patterns are consistent with the dissimilar tectonic conditions across Antarctica, whereby East Antarctica
represents a stable continental craton underlain by thick continental
lithosphere (Heeszel et al. 2011) and West Antarctica is composed
of crustal blocks that have undergone Mesozoic and Cenozoic tectonism, with a relatively hot, shallow mantle asthenosphere. Potentially, the disparate splitting patterns reflect large-scale deflection
of mantle flow around the East Antarctic craton. This mechanism
has been suggested for other locations in close proximity to cratons
(e.g. Clitheroe & van der Hilst 1998; Walker et al. 2004; Miller
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The motion of plates over the mantle may induce asthenospheric
shear strain beneath the plate with the axis of shear aligned in
the direction of plate motion relative to the ambient mantle (e.g.
Wang et al. 2008). This plate basal shear mantle deformation mechanism should cause seismic anisotropy such that the olivine fast
axis becomes oriented in the direction of plate motion in an absolute mantle reference frame, thus producing fast anisotropy directions parallel to absolute plate motion (APM; e.g. Savage 1999).
However, the fast anisotropic directions from the splitting measurements consistently show large angles with APM directions in both
no-net-rotation (NNR) and hotspot (HS) absolute reference frames
(Argus et al. 2010; Fig. 3). The NNR frame represents the plate velocity relative to the weighted average of all the plate velocities on
the earth’s surface (Argus & Gordon 1991). In comparison, the HS
reference frame minimizes the movement of hotspots, and its use as
an absolute reference frame assumes that hotspots are fixed relative
to the lower mantle (e.g. Minster et al. 1974). The inferred basal
shear direction for both HS and NNR absolute reference frames is
highly inconsistent with most of the observed splitting results, with
the exception of a few far Grid west sites (e.g. THUR and DNTW).
This suggests that the anisotropic fabric does not broadly result
from shear associated with the motion of Antarctic lithosphere over
the mantle. The lack of signal from Antarctica motion relative to the
mantle is not surprising seeing that Antarctica moves slowly in both
the NNR (1.77 cm yr−1 in a direction of 91.2◦ E) and HS (1.89 cm
yr−1 in a direction of 281.6◦ W) reference frames. Debayle & Yanick
(2013) note that only fast moving plates (velocity >4 cm yr−1 ) produce sufficient shearing at their base to organize anisotropy within
the asthenosphere beneath the entire tectonic plate. Further, beneath
slow moving plates (like Antarctica), plate motion is predicted to
only partially control mantle asthenospheric flow, given that the
uppermost mantle is subject to other secondary convection mechanisms.
Seismic anisotropy beneath W. Antarctica
et al. 2013). In this scenario, asthenospheric flow is directed around
the edge of the cratonic keel rather than beneath it, leading to fast
directions oriented approximately parallel to the craton edge (Fouch
et al. 2000; Miller & Becker 2012). However, splitting results from
the boundary of the East Antarctic craton are oriented normal rather
than parallel to the margin, suggesting that simple edge driven flow
cannot explain the observations. Further, the small magnitude APM
reported for Antarctica and absence of vigorous mantle flow due to
large-scale tectonics likely results in insufficient shearing within the
asthenosphere needed to coherently deflect flow around the cratonic
keel (Miller & Becker 2012; Miller et al. 2013). These observations
suggest that, as is observed in other cratonic regions, the splitting of East Antarctica results largely from anisotropy frozen into
thick and cold continental lithosphere (e.g. Barklage et al. 2009),
whereas splitting seen in most of West Antarctica largely results
from asthenospheric fabric that reflects recent and ongoing mantle
processes. As discussed below, the same interpretation may not be
valid for the EWM crustal block.
5.4 WARS anisotropy resulting from Cenozoic extension
5.4.1 Extension induced mantle anisotropy
It is well accepted that rift systems develop via extension and ultimately rupture of thick continental lithosphere, however, the mechanisms that control the character and style of rifting remain contentious. Specifically, a new model of rift development in which
magmatic products (i.e. dikes and magma-filled lenses) accommodate a considerable proportion of extension (e.g. Buck 2004; Thybo
& Neilson 2009; Bialis et al. 2010) has been offered up in contrast to previous models of passive rifting (i.e. extension driven by
far field stresses). Simple 2-D orthogonal extension in the mantle
lithosphere associated with plate stretching should produce olivine
LPO and shear wave azimuthal anisotropy in the direction of extension (McKenzie 1979; Blackman et al. 1996; Vauchez et al. 2000).
However, due to the conflicting effects of strain accumulation and
lithospheric thinning, this signal is expected to be weak. Extensionparallel fast directions have been reported for multiple continental
rift systems (Vinnik et al. 1992; Gao et al. 1997) and portions of
the Basin and Range (Obrebski et al. 2006; Xue & Allen 2006),
one of the few continental extensional systems of similar size and
scale to the WARS. Studies at oceanic spreading centres also report
extension parallel fast axes which are likely due to flow induced
LPO of anisotropic minerals within the asthenospheric mantle (e.g.
Blackman & Kendall 1997; Wolfe & Solomon 1998).
Conversely, if extension is accommodated largely by magmatic
intrusion, splitting fast directions will align parallel to strike of the
bodies and hence be rotated 90◦ to the direction of shearing. Studies
of the Main Ethiopian Rift, the northernmost portion of the East
African rift system (EARS), reveal shear-wave-splitting directions
aligned parallel to the rift axes indicative of the presence of oriented
melt pockets in the uppermost mantle and crust (Walker et al. 2004;
Kendall et al. 2005) or alternatively indicating large-scale, oriented
mantle flow in the presence of a superplume (Bagley & Nyblade
2013).
Recently, Eilon et al. (2014) reported extension parallel fast axes
with large delay times (>1 s) from the highly extended continent
within the Woodlark Rift. They suggest that the anisotropic signal
there represents extension driven LPO of olivine within the asthenospheric mantle similar to mechanisms inferred at mid-ocean ridges.
Further, they propose that the anisotropic fabric within highly extended continental rift regimes may characterize a transition from
small-strain continental rifts (where fast axes may be expected to
parallel the strike of melt bodies and hence the rift axis) to midocean ridges (where fast axes parallel the extension direction).
5.4.2 Anisotropic signature of Cenozoic WARS extension
Rifting within the Ross Sea sector of the WARS during the Eocene
and Oligocene was multiphase with extension occurring largely
transcurrently with isolated periods and locations of orthogonal extension (Wilson 1995; Miller et al. 2001, Granot et al. 2013). Poles
of East Antarctica–West Antarctica rotation for that time period
(40–26 Ma) are located in central West Antarctica and suggest that
the western sector of the WARS and specifically the locations of
the remarkably deep subglacial basins (i.e. Pine Island Rift and the
Bentley Trench) underwent a significant period of oblique convergence (Granot et al. 2013). These poles predict roughly east-west
convergence in the vicinity of the POLENET transect, which is at
large angles to the observed fast directions. This indicates that the
anisotropy is not correlated with the inferred transcurrent and convergent deformation during Eocene–Oligocene time in the central
and southern WARS.
Limited extension within the Adare Basin (northern WARS),
coeval with the opening of the Terror Rift (Henrys et al. 2007;
Fielding et al. 2008) during the mid-Miocene, suggests that a major
change in relative plate motion between East and West Antarctica
occurred at ∼17 Ma. This change in tectonic framework is proposed
to be concomitant with the final pulse of extension within the WARS
where rifting increased towards the interior of the WARS (Granot
et al. 2010). The WARS extension direction during this final pulse
of rifting is not known, as magnetic anomalies in the Adare Trough
do not allow a pole of rotation to be calculated for this time period
(Granot et al. 2010). However, it is a reasonable assumption that
the extension would have been orthogonal or at high angle to well
developed rift structures in the Pine Island Glacier basin and Bentley
Trench.
Fast directions along and near the POLENET transect in the
WARS are seen to indeed be oriented approximately orthogonal
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Figure 6. Antarctica, showing splitting vectors from this and a variety of
other studies (Müller 2001; Bayer et al. 2007; Usui et al. 2007; Reading &
Heinz 2008; Barklage et al. 2009; Hernandez et al. 2009). Only results from
stations with multiple high-quality observations were plotted. Background
colour scale indicates bedrock elevation from Fretwell et al. 2013.
423
424
N. J. Accardo et al.
to the topographic axis of the Bentley Trench (Fig. 4). The largest
splitting time (1.43 s) occurs at station ST04, which is located above
the Bentley Trench and also shows the smallest crustal thickness
(21 km) and greatest amount of crustal thinning along the transect
(Chaput et al. 2014). The delay times decrease coast-wards along
the transect towards MBL, consistent with higher elevation, thicker
crust and decreasing extension (Chaput et al. 2014). This preliminarily suggests that the anisotropy across this region results from
deformational fabric accrued during the last (Neogene) phase of
strong WARS rifting, potentially focused within the Bentley Trench.
Splitting measurements towards the Pine Island Glacier region
(UPTW, DNTW, BEAR, THUR) do not show a similar correspondence between fast directions and the inferred extension direction.
This may be partly due to the complex tectonic history in this region;
Eocene-Oligocene rotation poles predict increasing convergence in
this direction and Granot et al. (2013) proposed that Pine Island
and other present rift basins were transform fault features at that
time. Some of the fast directions (e.g. DNTW) are oriented approximately in the expected shear direction of these transform faults.
Alternatively, fast directions in this region may be related to radial
flow associated with the MBL plume, as discussed in Section 5.6.
If continental extension produces mantle anisotropy with strong
extension-parallel fast directions in West Antarctica, an important
question is why are similar extension parallel orientations not found
at some other prominent continental rift zones such as the EARS?
Along-strike fast directions in the EARS are hypothesized to be due
to SPO anisotropy due to melt-filled extensional cracks (Walker
et al. 2004; Kendall et al. 2005) and/or to large-scale along-strike
flow from the African superplume (Bagley & Nyblade 2013). The
African superplume explanation suggests that perhaps the East
Africa observations cannot be generalized to other continental rifts.
Melt-filled cracks are certainly a possibility in any extensional region, but most evidence suggests that the WARS is not currently
undergoing significant extension (Donnellan & Luyendyk 2004;
Winberry & Anandakrishnan 2004; Wilson et al. 2011), whereas
the East African Rift is currently extending at rates of up to 6 mm
yr−1 (Stamps et al. 2008).
5.5 Anisotropy resulting from the rotation
of the Ellsworth-Whitmore crustal block
Splitting parameters in the EWM are consistent with results from
the adjacent stations in the WARS potentially suggesting that the
same extensional mantle deformational fabric producing the strong
anisotropy in the WARS extends beneath the EWM block. However,
mantle shear velocities in the region are higher beneath the EWM
than the rest of West Antarctica, suggesting the EWM, as indicated
by the geology and palaeomagnetic data, may be underlain by older
continental lithosphere not present within the WARS (Heeszel et al.
2011; Lloyd et al. 2013; Fig. 7). Further, mantle seismic velocities
found in the EWM are significantly lower than those for the East
Antarctic craton or even the PCM region, suggesting nonetheless
some tectonic modification of the continental lithosphere. Combined this evidence suggests that the translocation and rotation of
the EWM likely induced additional deformation and alteration of
the continental lithosphere compared to the surrounding regions.
Alternatively, splitting parameters in the EWM region may reflect
deformation due to shear motion within the long inferred (Storey &
Dalziel 1987) and recently aeromagnetically imaged Pagano Shear
Zone (PSZ), a major left-lateral strike-slip fault system between East
and West Antarctica (Jordan et al. 2013; Fig. 7). The PSZ strikes
approximately Grid NE-SW at the juncture of the EWM and the
TAM. Elongate, structurally controlled Jurassic granite intrusions
along the flanks of the PSZ suggest that the fault system accommodated motion primarily during the Mesozoic. Gravity and magnetic
anomaly mapping show no evidence for a connection between the
Mesozoic rifting events and the Cenozoic WARS rifting, although
the Early Jurassic Ferrar-Karoo Large Igneous Province suggests
rifting in the WARS may have originated at that time. Additionally, shear velocity maps reveal that the PSZ boundary, imaged by
Jordan et al. (2013), sits exactly along the boundary between the
fast velocities in East Antarctica and the slow velocities in West
Antarctica (Fig. 7). Results from ST01, located within the PSZ,
show fast axes subparallel to the proposed strike of the PSZ and
dissimilar from observations along the remainder of the POLENET
transect (Fig. 5). Thus, anisotropic fabric beneath the PSZ may be
Downloaded from http://gji.oxfordjournals.org/ at The University of Montana on September 16, 2016
Figure 7. S-wave velocity variation at 180 km depth (after Heeszel et al.
2011) with splitting parameters from this study in black and previous studies
(Müller 2001; Barklage et al. 2009; Hernandez et al. 2009) in white. The
boundary of the Pagano Shear Zone (PSZ; Jordan et al. 2013) are represented
by the purple solid and dashed lines; solid line indicates where the PSZ has
been identified from aerogeophysical data, dashed line indicates inferred
continuation of the PSZ towards West and East Antarctica, respectively. The
lowest velocity region directly underlies the MBL dome, and the highest
velocities are found across the TAM and into the East Antarctic craton.
We alternatively suggest that the anisotropic signature observed within the WARS was jointly imparted to the lithosphere
and asthenosphere via ductile shearing associated with the largemagnitude extension between East and West Antarctica. Deformation within the lithosphere and asthenosphere leading to the accumulation of pervasive anisotropy likely dominated over competing
mechanisms like lithospheric thinning which would have otherwise
acted to obstruct the retention of a coherent anisotropic fabric. This
is in good agreement with the study of Eilon et al. (2014) which
suggested that extension parallel fast axes within the highly extended Woodlark Rift in Papua New Guinea represent widespread
coherent anisotropic fabric within the asthenosphere accumulated
via extension controlled LPO. They point out that the observation
of extension parallel fast axes with large delay times (>1 s) beneath
highly extended continental rifts represent mantle anisotropic fabrics dominated by flow produced LPO rather than fabrics characterized by melt or pre-existing structure. We suggest that the anisotropy
observed within the WARS resulted from extensive Cenozoic extension including the final pulse of western WARS rifting in the
Miocene, the signature of which has since been preserved within
the asthenospheric mantle owing to the weak influence of shear
from small-magnitude APM associated with Antarctica.
Seismic anisotropy beneath W. Antarctica
5.6 Anisotropy resulting from a mantle plume
MBL exhibits splitting times and directions that depart systematically and are locally discordant with observations within the WARS
and TAM. Mantle shear velocity maps highlight an upper mantle low velocity region associated with the MBL, and suggest that
splitting in this region may represent processes associated with a
MBL mantle plume (Heeszel et al. 2011). Models for anisotropy
at mantle plumes suggest that fast anisotropic directions should
be oriented vertically within the central upwelling and radially
within the expanding plume head (e.g. Rümpker & Silver 2000;
Xue & Allen 2005). With the superimposed influence from absolute plate motion, horizontal flow away from the central plume
head upwelling is predicted to be parabolic (Walker et al. 2005).
Conversely, recent experimental studies suggest that despite radially outward flow of material within the plume head, olivine fast
axes will coherently align perpendicular to the flow in an azimuthal
pattern. Specifically, it is proposed that the interchange between
radial shortening and azimuthal stretching within the expanding
plume head induces extensional pure strain, thus locking the crystalline alignment into a flow normal orientation (Druken et al.,
in preparation).
Anisotropy within the central upwelling of the MBL hot spot
should be weak with small delay times and varied fast directions
due to the vertical orientation of the olivine crystals. Splitting above
the eastern MBL dome is indeed small in magnitude (delay times
average ∼0.6 s) in comparison to those observed in the WARS yet
shows a consistent fast direction (e.g. ST13, CLRK, ST08) of ∼40◦ .
Mantle xenoliths from MBL indicate that portions of the sampled
lithospheric mantle have minimum ages of 1.3–1.5 Ga (Handler
et al. 2003). Thus, the thicker MBL crust and/or the preservation of
MBL Proterozoic lithosphere implies that consistent fast directions
in this region may represent an older signature of anisotropy that
has been ‘frozen into’ the lithospheric mantle prior to the onset of
plume activity and Mesozoic-Cenozoic extension.
Anisotropy outside of the proposed plume axis, but influenced by
plume head processes should show fast axes oriented either parallel
or normal to radial flow from the proposed plume head. However,
only partial indications of a radially or flow normal oriented pattern
of fast directions is observed. Results from the Amundsen Sea
region (BEAR, DNTW, UPTW, WAIS) are consistent in delay time
(∼0.75 s) and have fast directions that are approximately radially
oriented with respect to the central MBL dome. Shear velocity maps
of the region at depth further emphasize the spatial relationship
between the radial anisotropic pattern and the low upper mantle
velocities extending Grid northwest from the centre of the MBL
dome (Heeszel et al. 2011; Fig. 7). However, this radial pattern of
anisotropic directions is not reported at stations Grid southeast of
MBL (e.g. CLRK, MPAT), which display fast directions that are
subparallel to those observed along the central TAM and within the
WARS, where upper mantle shear velocities are somewhat higher.
6 C O N C LU S I O N S
Core phase-derived splitting results from West Antarctica generally
show a large number of stations with distinct (∼1 s) anisotropy and
fast axes oriented Grid NE–SW, orthogonal to the trend of the TAM.
We suggest that anisotropy in this region is strongly influenced by
mantle fabrics that were established during Cenozoic WARS extension, crustal thinning and associated mantle flow. Specifically, splitting along the relatively dense transect of POLENET/ANET seismic
stations within the WARS (between the MBL dome and the EWM) is
subparallel to the apparent direction of Neogene WARS extension,
indicating the absence of partial melt or other influences within
the rift system that would result in a rift axis parallel or oblique
fast direction trends. Rather, plate stretching within the highly extended WARS led to the accumulation of pervasive anisotropic fabrics within the uppermost mantle and dominated over competing
processes like lithospheric thinning which would have otherwise
hindered the retention of a widespread, coherent anisotropic signal.
The weak influence of shear from small-magnitude APM likely allowed for the preservation of such widespread coherent fabric after
WARS extension shut down.
Multiple hypotheses exist to explain the pattern of splitting
within the EWM crustal block. Broadly the EWM microplate shows
anisotropy that is consistent with the WARS, suggesting either coincident underlying alignment with WARS trends or underlying mantle that has participated in WARS-oriented flow. Alternatively, the
anisotropic fabric may be related to the deformation within the PSZ
between the East Antarctic craton and the EWM block or perhaps
be evidence for fabric frozen into the mantle lithosphere prior to
fragmentation of the Gondwanaland supercontinent. Although the
observations are ambiguous, weaker splitting parameters observed
at the MBL dome are compatible with less pervasive Cenozoic extension and somewhat thicker lithosphere, or with variable splitting
associated with upper mantle plume and plume head influences.
Fast directions in the Amundsen Sea region are approximately radial to the MBL dome and may indicate a mantle flow pattern away
from the dome, or less clearly, from the complicated Cenozoic tectonics associated with Eocene-Oligocene convergence followed by
Neogene extension responsible for the formation of extremely deep
subglacial basins and troughs in the region.
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representative of Mesozoic shearing from this feature. In contrast,
in the simplest interpretation, anisotropy within the larger EWM
results from mantle deformation associated with the same Neogene
large-scale extensional processes that produced the WARS which
is either coincidentally aligned with, or has been sufficient to overprint, prior deformation associated with the tectonic history of the
block.
Additionally, a third possibility exists. While details of the tectonic history of the EWM remain enigmatic it is now largely accepted that this tectonic block originated adjacent to the South
African Cape Fold Belt prior to Gondwana breakup (e.g. Dalziel
2007) and has translated and rotated as a microplate. In this scenario,
the EWM crustal block rotated as much as 90◦ counterclockwise
relative to East Antarctica (Grunow et al. 1987; Randall & Niocaill
2004) and was laterally translated to its present day position at the
head of the Weddell Sea embayment. Fast anisotropic directions in
this crustal block are consistently oriented at approximately Grid
25◦ , similar to splitting directions in the WARS, and systematically
different from results in the PCM. The EWM splitting directions
may represent anisotropy frozen into the continental lithosphere of
the EWM block, which would have originally been parallel with
PCM lithospheric anisotropy. The azimuths of the splitting directions in the EWM crustal block and in the PCM are in each case a
few degrees clockwise with respect to the strike of the early Mesozoic Gondwanide structural trends. This suggests that restoration
of the EWM crustal block within a reconstruction of the Gondwana supercontinent would bring them into parallelism, implying
the anisotropy in the EWM crustal block may have been ‘frozen’
into the lithosphere prior to breakup of the supercontinent.
425
426
N. J. Accardo et al.
AC K N OW L E D G E M E N T S
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S U P P O RT I N G I N F O R M AT I O N
Additional Supporting Information may be found in the online version of this article:
Figure S1. Individual A and B quality splitting results for all events
stacked in the final solutions for all stations used in this study.
Table S1. A summary of individual splitting results for all events
stacked in the final solutions for all stations used in this study.
Parameters listed include the station of interest, the event time,
the core-phase used, the SNR for the radial (R-SNR), transverse
(T-SNR), and vertical (Z-SNR) components, the back-azimuth for
the event-station pair (BAZ), the polarization azimuth (PAZ), the
orientation of the fast axis, the magnitude of the delay time (Lag),
and the graded quality of the measurement (Grade).
(http://gji.oxfordjournals.org/lookup/suppl/doi:10.1093/gji/
ggu117/-/ DC1).
Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the
authors. Any queries (other than missing material) should be directed to the corresponding author for the article.
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